1. Field of the Invention
This invention relates to low cost, low phase noise oscillators. The oscillator of this invention is designed to be used in many applications where low phase noise is a main design consideration.
2. Description of Related Art
There are many oscillating circuits available for use at high frequencies and each has certain performance limitations. The well known Colpitts oscillator works well up to about 1 Ghz. Above 1 Ghz the configuration generally referred to as the standard microwave oscillator is used and works very well as a low cost, medium phase noise voltage controlled oscillator. The standard configuration does not exhibit good phase noise performance, even with a high Q resonator.
A low phase noise oscillator for operation at frequencies between 400 MHz and 6 Ghz is disclosed in U.S. Pat. No. 5,748,051 to Lewis, the substance of which is in corporated herein by reference in its entirety. Other high frequency oscillators include a modulated transistor oscillator disclosed in U.S. Pat. No. 3,855,553 to Cronin, a television tuner oscillator with feedback for more low frequency power disclosed in U.S. Pat. No. 4,564,822 to Saitoh et al., a low noise oscillator disclosed in U.S. Pat. No. 4,580,109 to Lockwood, a varactor voltage-controlled UHF oscillator Disclosed in a German Patent Number 28 11 080 A modified Clapp oscillator is disclosed in British Patent 11,114,023 to Michalek.
Other circuits dealing with phase noise problems include a negative resistance oscillator with electronically tunable base inductance described in U.S. Pat. No. 5,373,264 to Higgins Jr., a low noise oscillator which suppresses phase noise caused by internal noise of the oscillator disclosed generally in U.S. Pat. No. 5,337,014 to Najle et al. for a circuit for phase noise measurements in U.S. Pat. No. 5,341,110 to Nardi for a low phase noise reference oscillator, and in U.S. Pat. No. 5,374,902 to Bradley for an ultra low phase noise microwave synthesizer.
The present invention is concerned with an oscillator circuit for use in UHF and microwave applications which require the generation of low phase noise signals. More specifically, the present invention is directed to an oscillator circuit which includes a band pass filter having an inductor in series with a low value capacitor. This series combination of an inductor and a capacitor, in parallel with a low value inductor forms a resonator whose inductive and capacitive reactances are a very low value. This causes the loaded Q to be very close to the resonator""s unloaded Q, thereby resulting in significantly reduced phase noise.
In accordance with the invention, the phase shift of the band pass filter network is the negative of the phase shift of the circuit""s transistor so as to make the overall phase shift equal to zero at the oscillation frequency. The insertion loss of the band pass filter is small enough as to make the overall gain greater than unity.
A goal of this invention is to minimize the oscillator""s phase noise, which depends on the loaded Q of the circuit. The subject low phase noise oscillator is superior to the UHF and microwave oscillators presently in use because this circuit takes better advantage of the resonator""s Q.